--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -4176,38 +4176,16 @@ function toOpenWindowByType(inType, uri,
   else
     window.open(uri, "_blank", "chrome,extrachrome,menubar,resizable,scrollbars,status,toolbar");
 }
 
 function OpenBrowserWindow(options) {
   var telemetryObj = {};
   TelemetryStopwatch.start("FX_NEW_WINDOW_MS", telemetryObj);
 
-  function newDocumentShown(doc, topic, data) {
-    if (topic == "document-shown" &&
-        doc != document &&
-        doc.defaultView == win) {
-      Services.obs.removeObserver(newDocumentShown, "document-shown");
-      Services.obs.removeObserver(windowClosed, "domwindowclosed");
-      TelemetryStopwatch.finish("FX_NEW_WINDOW_MS", telemetryObj);
-    }
-  }
-
-  function windowClosed(subject) {
-    if (subject == win) {
-      Services.obs.removeObserver(newDocumentShown, "document-shown");
-      Services.obs.removeObserver(windowClosed, "domwindowclosed");
-    }
-  }
-
-  // Make sure to remove the 'document-shown' observer in case the window
-  // is being closed right after it was opened to avoid leaking.
-  Services.obs.addObserver(newDocumentShown, "document-shown");
-  Services.obs.addObserver(windowClosed, "domwindowclosed");
-
   var handler = Components.classes["@mozilla.org/browser/clh;1"]
                           .getService(Components.interfaces.nsIBrowserHandler);
   var defaultArgs = handler.defaultArgs;
   var wintype = document.documentElement.getAttribute("windowtype");
 
   var extraFeatures = "";
   if (options && options.private) {
     extraFeatures = ",private";
@@ -4242,16 +4220,20 @@ function OpenBrowserWindow(options) {
 
     // we should "inherit" the charset menu setting in a new window
     win = window.openDialog("chrome://browser/content/", "_blank", "chrome,all,dialog=no" + extraFeatures, defaultArgs, charsetArg);
   } else {
     // forget about the charset information.
     win = window.openDialog("chrome://browser/content/", "_blank", "chrome,all,dialog=no" + extraFeatures, defaultArgs);
   }
 
+  win.addEventListener("MozAfterPaint", () => {
+    TelemetryStopwatch.finish("FX_NEW_WINDOW_MS", telemetryObj);
+  }, {once: true});
+
   return win;
 }
 
 /**
  * Update the global flag that tracks whether or not any edit UI (the Edit menu,
  * edit-related items in the context menu, and edit-related toolbar buttons
  * is visible, then update the edit commands' enabled state accordingly.  We use
  * this flag to skip updating the edit commands on focus or selection changes